The invention concerns a transmission for commercial vehicles.
Together with the service brakes of a vehicle, especially of a commercial vehicle, which as a rule are friction brakes subject to wear, additional deceleration devices such as retarders are being increasingly required by law and offered by vehicle manufacturers. A wear-free brake is required, particularly in city autobuses, but also in other vehicles, in order to improve operating costs and operating safety.
Belonging to retarders are hydrodynamic, hydrostatic or electrodynamic braking devices situated on both the transmission or engine and those systems which are provided within the transmission housing. Moreover, retarders are separated into primary retarders, which work depending on the engine speed, and secondary retarders, which work depending on the vehicle speed. Retarders for automatic transmissions of commercial vehicles, having planetary gear sets, are mainly operated as primary retarders. Such arrangements are known, for example, from the article "Retarder rechnen sich" in the periodical "Nutzfahrzeug", December 1991, pp. 32 ff.
Different methods are distinguished for actuating the retarder. The braking action is controlled in stages or continuously within the scope of the maximum operational efficiency of the retarder.
Motor vehicles with internal combustion engines are started by different methods:
with a mere speed converter (starting clutch dry or wet); PA0 with a speed and torque converter (triloco converters) of different designs; PA0 geared-neutral by transmissions having infinite ratio or PA0 by hydrostatic or electromotor transmissions.
In autobuses having automatic transmissions, hydrodynamic converters are at present used almost without exception.
In the past, a torque converter was often sought to be used wear-free brake (retarder), for example, it has been described by Johannes Looman, "Zahnradgetriebe", page 310, 3rd edition 1996, Springer-Verlag, that holding the stator of a torque converter of an automatic transmission, for omnibuses, stationary lets the converter act as a retarder. In the same book, page 305, this was also similarly described for another automatic transmission for commercial vehicles where the converter was also used at the same time as hydrodynamic brake.
Such retarders have, however, disadvantages of difficult control of the braking power and of poor braking power under certain circumstances.
The problem on which the invention is based is to simplify the structural design and to reduce the number of parts in an automatic transmission.
According to the invention the problem is solved by using a wear-free braking element for starting instead as well as a starting element being used as a retarder.
For this purpose a hydrodynamic retarder is combined with a planetary gear set.